improving the overall stereo ##sel ##ect ##ivity . these have been successful to varying degrees . additionally , modifications focused on altering the progress of the reaction , either by generating the pen ##tad ##ien ##yl cat ##ion in an uno ##rth ##od ##ox fashion or by having the ox ##yal ##ly ##l cat ##ion " intercepted " in various ways . furthermore , en ##ant ##ios ##ele ##ctive variants of various kinds have been developed . the sheer volume of literature on the subject prevents a comprehensive examination of this field ; key examples are given below . = = = silicon @ - @ directed cy ##cl ##ization = = = the earliest efforts to improve the select ##ivity of the na ##zar ##ov cy ##cl
improving the overall stereoselectivity. these have been successful to varying degrees. additionally, modifications focused on altering the progress of the reaction, either by generating the pentadienyl cation in an unorthodox fashion or by having the oxyallyl cation " intercepted " in various ways. furthermore, enantioselective variants of various kinds have been developed. the sheer volume of literature on the subject prevents a comprehensive examination of this field ; key examples are given below. = = = silicon @ - @ directed cyclization = = = the earliest efforts to improve the selectivity of the nazarov cycl